1. Field of the Invention
The present invention relates to a wire dot printer head and a wire dot printer.
2. Description of the Prior Arts
There has been known a wire dot printer head wherein an armature with a printing wire connected thereto is pivoted between a printing position and a stand-by position, and when the armature is pivoted to the printing position, a tip of the wire is brought into collision with a printing medium such as a paper to effect printing. In a certain wire dot printer head of this type, there has been proposed a device wherein a magnetic flux is produced by a coil around the armature to be pivoted for forming a magnetic circuit that causes the armature to be attracted from a stand-by position to a printing position to effect printing (see Japanese Unexamined Patent Publication No. 191036/1991). In the patent document 1, a yoke or the like for forming a magnetic circuit is formed by sintering Fe particles or Co particles having a fine particle diameter to thereby improve magnetic characteristic such as a saturated flux density or the like.
However, the improvement in the saturated flux density of only a yoke does not mean the improvement in the magnetic flux characteristic of the whole wire dot printer head as disclosed in the Japanese Unexamined Patent Publication No. 191036/1991. Specifically, it is necessary to prevent a flux loss among the components forming the magnetic circuit such as a yoke, armature or the like. Even in case where an armature spacer for forming a side magnetic path to the armature is provided in addition to the yoke or armature, in particular, it is important to prevent the magnetic saturation among these components.
On the other hand, in case where a pivot shaft is inserted in a through hole in which the armature is pivotably mounted with the pivot shaft as a center, the inner face of the through hole comes in contact with the pivot shaft to thereby be scraped. Therefore, a certain surface hardening process is required to be provided on the inner face of the through hole. However, when the surface hardening process is provided also on the inner face of the through hole, the magnetic flux is hard to transmit through this section (flux loss), thereby deteriorating the magnetic characteristic.
As described above, the magnetic characteristic is deteriorated by the flux loss among the components or flux loss caused by the surface hardening process on the through hole, so that magnetic characteristic required for high-speed printing cannot be obtained. Therefore, high-speed printing cannot be executed. In particular, the armature is required to be pivoted 2500 times per second between the printing position and the stand-by position with a recent increased printing speed. Therefore, the deterioration in the magnetic characteristic becomes an important problem.